Electrocardiography is the interpretation of electrical activity in the heart, as measured by electrodes placed on the skin. Voltages measured between different electrodes undergo minute variations as a function of time, reflecting various stages of the operation of the heart. A recording or display of the voltages as a function of time is called an electrocardiogram, often abbreviated ECG or EKG.
To record a typical ECG, a number of electrodes are placed on a patient's skin, at least some of the electrodes on opposing sides of the patient's cardiac divide. As many as 10 or more electrodes may be used. Voltages measured between different pairs of electrodes are called “leads”, and as many as 12 or more leads may be displayed or recorded in a complete ECG. However, meaningful analysis may be performed with only a handful of electrodes, even as few as two, in which case only one lead is available for recording.
An examination of the ECG can reveal valuable information about the functioning of the heart. For example, FIG. 1 illustrates a simplified single lead ECG 100 depicting a normal sinus rhythm, which is the normal beating of the heart. Each beat of the heart results in a complex of peaks and valleys, also called deflections, in the ECG, labeled P, Q, R, S, and T. The distance between the complexes reflects the time between heart beats, and therefore the heart rate, which is typically about 60-80 beats per minute for a resting patient, but depends somewhat on the patient's age, overall health, fitness level, and other factors.
Arrhythmias occur when the heart departs from the normal sinus rhythm, for example by beating unusually rapidly, unusually slowly, irregularly, or in an unsynchronized manner. Arrhythmias or other heart abnormalities may be diagnosed by examination of an ECG. For example, during atrial fibrillation, a common arrhythmia, the P deflections may be missing from the ECG. Many other specific ECG patterns have been identified and associated with particular arrhythmias.
Arrhythmias are often intermittent. For example, the heart may stay in a normal sinus rhythm for long periods between brief periods of arrhythmia. A single patient may also exhibit different kinds of arrhythmias at different times, or at the same time. If no filtering or data reduction were utilized, diagnosing intermittent arrhythmias would require recording a very long ECG and then laboriously scanning the ECG for periods of abnormality.
Automated ECG monitoring and identification of arrhythmias may be performed, and it is desirable that the results of such monitoring be presented in an efficient and complete manner.